1. Technical Field
The present invention relates to a compression/encryption device for carrying out data compression and a decompression/decryption device corresponding to the compression/encryption device.
2. Related Art
Recently, the use of image data has become more and more common with the spread of digital cameras and scanned electronic documentation. There has also been a trend towards high definition image data, as a result of which the data size for a single image has tended to increase.
On the other hand, with image data there is an accompanying risk of data disclosure due to interception on the internet or as a result of being viewed by unauthorized persons. In order to deal with this type of problem, a method of encrypting image data is effective. For example, if encryption is carried out using an en encryption method such as triple DES or AES, data integrity is considered to be protected.
However, there is a problem that the amount of processing expended by these safe encryption methods results in a significantly increased processing time (or CPU load) in proportion to the data size. The same is true for decryption methods. It also goes without saying that as the number of data items increases, the processing time is increased in proportion.
Addressing this type of problem, for example, Japanese Patent Laid-open No.2000-115551 discloses an encryption method that reduces the amount of processing involved in encryption by dividing a single image into a plurality of blocks, encrypting only some blocks, and computing a difference from the encrypted blocks for the remaining blocks.
Also, Japanese Patent Laid-open No. Hei. 6-125553 discloses an encryption method that reduces the amount of processing involved in encryption by only encrypting a direct current component when carrying out DCT (Discrete Cosine Transform) transformation on the image data.
Further, Japanese Patent Laid-open No. 2002-190798 discloses an encryption method that reduces the overall processing load involved in encryption by carrying out encryption using strong encryption means for parts of the data having high importance based on predetermined analysis rules, and then carrying out weak encryption processing on other sections of the data.
However, the method disclosed in Japanese Patent Laid-open No.2000-115551 only carries out reliable encryption processing on part of the image data to be encrypted, while it is also necessary to compute a difference between remaining portions and the encrypted portions, which means that for a large amount of data the overall processing amount will not always decrease.
Moreover, with the method disclosed in Japanese Patent Laid-open No. Hei 6-125553, since a dc component occupies a constant amount of the overall data (normally 1/64), there is a problem that the amount of processing involved in encryption is also increased in proportion to the data size.
Further, since the method disclosed in Japanese Patent Laid-open No. 2002-190798 can only be applied to data that is defined as a predetermined priority in each section of the image, there is a problem that the data the method can be applied to is limited.
As can be seen, in the related art, there is a problem in that the processing load (or processing time) for the amount of processing expended on encryption processing increases as the data size increases, and that for unknown formats there is no effect.
Encryption of image data has been given above by way of example, but the same problems also apply to data other than image data.